docs: def parse(self, buffer):
          """This method is used to parse an existing datagram into its
          corresponding TftpPacket object. The buffer is the raw bytes off of
          the network."""
          log.debug("parsing a %d byte packet" % len(buffer))
          (opcode,) = struct.unpack(str("!H"), buffer[:2])
          log.debug("opcode is %d" % opcode)
          packet = self.__create(opcode)
          packet.buffer = buffer
          return packet.decode()